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SubjectRe: [PATCH v9 2/3]: perf record: enable asynchronous trace writing
On Fri, Oct 05, 2018 at 11:31:11AM +0300, Alexey Budankov wrote:
> Hi,

Hi,

>
> On 05.10.2018 10:16, Namhyung Kim wrote:
> > On Wed, Oct 03, 2018 at 07:12:10PM +0300, Alexey Budankov wrote:
> <SNIP>
> >> +static void record__aio_sync(struct perf_mmap *md)
> >> +{
> >> + struct aiocb *cblock = &md->cblock;
> >> + struct timespec timeout = { 0, 1000 * 1000 * 1 }; // 1ms
> >> +
> >> + do {
> >> + if (cblock->aio_fildes == -1 || record__aio_complete(md, cblock))
> >> + return;
> >> +
> >> + while (aio_suspend((const struct aiocb**)&cblock, 1, &timeout)) {
> >> + if (!(errno == EAGAIN || errno == EINTR))
> >> + pr_err("failed to sync perf data, error: %m\n");
> >
> > Is there somthing we can do in this error case? Any chance it gets
> > stuck in the loop?
>
> Not really. Currently, in glibc, it can block on a mutex only.

OK, I was thinking of a situation where the aio_suspend() keeps
returning a same error code repeated..


>
> >
> >
> >> + }
> >> + } while (1);
> >> +}
> >> +
> >> +static int record__aio_pushfn(void *to, struct aiocb *cblock, void *bf, size_t size)
> >> +{
> >> + off_t off;
> >> + struct record *rec = to;
> >> + int ret, trace_fd = rec->session->data->file.fd;
> >> +
> >> + rec->samples++;
> >> +
> >> + off = lseek(trace_fd, 0, SEEK_CUR);
> >> + lseek(trace_fd, off + size, SEEK_SET);
> >
> > It'd be nice if these lseek() could be removed and use
> > rec->bytes_written instead.
>
> Well, this could be implemented like this avoiding lseek() in else branch:
>
> off = lseek(trace_fd, 0, SEEK_CUR);
> ret = record__aio_write(cblock, trace_fd, bf, size, off);
> if (!ret) {
> lseek(trace_fd, off + size, SEEK_SET);
> rec->bytes_written += size;
>
> if (switch_output_size(rec))
> trigger_hit(&switch_output_trigger);
> }

Oh I meant the both like:

off = rec->bytes_written;
ret = record__aio_write(cblock, trace_fd, bf, size, off);
if (!ret) {
rec->bytes_written += size;

...


> >
> >
> >> + ret = record__aio_write(cblock, trace_fd, bf, size, off);
> >> + if (!ret) {
> >> + rec->bytes_written += size;
> >> +
> >> + if (switch_output_size(rec))
> >> + trigger_hit(&switch_output_trigger);
> >
> > Doesn't it need the _sync() before the trigger? Maybe it should be
> > moved to record__mmap_read_evlist() or so..
>
> Currently trigger just updates variable state.
> The state is then checked thru separate API at __cmd_record() where
> record__mmap_read_sync() is called prior switching to a new trace file
> or finishing collection.
>
> >>
> <SNIP>
> >> if (map->base) {
> >> +#ifndef HAVE_AIO_SUPPORT
> >> if (perf_mmap__push(map, rec, record__pushfn) != 0) {
> >> rc = -1;
> >> goto out;
> >> }
> >> +#else
> >> + if (!rec->opts.nr_cblocks) {
> >> + if (perf_mmap__push(map, rec, record__pushfn) != 0) {
> >> + rc = -1;
> >> + goto out;
> >> + }
> >> + } else {
> >> + /*
> >> + * Call record__aio_sync() to wait till map->data buffer
> >> + * becomes available after previous aio write request.
> >> + */
> >> + record__aio_sync(map);
> >> + if (perf_mmap__aio_push(map, rec, record__aio_pushfn) != 0) {
> >> + rc = -1;
> >> + goto out;
> >> + }
> >> + }
> >> +#endif
> >
> > If dummy aio functions are provided, the #ifdef can be removed and
> > just use the #else part assuming opts.nr_cblocks == 0.
>
> Yes, it looks a little bit cumbersome. Would this be more compact?

Why not exposing opts.nr_cblocks regardless of the #ifdef? It'll have
0 when it's not compiled in. Then it could be like below (assuming
you have all the dummy aio funcitons):


>
> if (map->base) {
> if (!rec->opts.nr_cblocks) {
> if (perf_mmap__push(map, rec, record__pushfn) != 0) {
> rc = -1;
> goto out;
> }
> } else {
> int idx;
> /*
> * Call record__aio_sync() to wait till map->data buffer
> * becomes available after previous aio write request.
> */
> idx = record__aio_sync(map, false);
> if (perf_mmap__aio_push(map, rec, idx, record__aio_pushfn) != 0) {
> rc = -1;
> goto out;
> }
> }
> }
>

Thanks,
Namhyung

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